1. Field of the Invention
The aspects of the present disclosure relate generally to power supplies for electroluminescent devices and in particular to resonant power converters used to drive light emitting diode arrays.
2. Description of Related Art
Light Emitting Diode (LED) arrays, in which multiple LEDs are formed into an array and powered as a unit, are gaining popularity in lighting and signaling applications. LED arrays are typically connected to a direct current (DC) power source where the amount of applied current controls the brightness of emitted light. Switched mode power supplies are often used to provide power for LED arrays and other appliances requiring low level direct current (DC) power. Switched mode power supplies generally relate to a class of voltage regulation apparatus where input DC power is chopped by a switching circuit and fed into an energy storage device, usually an inductor. Energy from the chopped DC power is alternately stored into a magnetic field and discharged therefrom into a secondary circuit containing rectification and filtering to produce a conditioned DC output voltage. A controller is typically used to monitor conditions of the output power and adjust the switching circuit accordingly to provide regulation of the output. Switched mode supplies fall into two categories. Supplies that convert DC to alternating current (AC) are known as ‘inverters’ and supplies that convert DC to DC are known as ‘converters’. A converter is typically an inverter followed by a rectifier.
A block diagram of a typical switched mode power source used to supply LED arrays is shown in FIG. 1. The switched mode power source 100 receives AC input power 102 which may be supplied from a local mains power source such as for example the 120 volt, 60 Hertz power available in the United States, 50 Hertz 230 volt power available in many European countries, or other suitable AC power sources. An input rectifier 110 is used to convert the AC input power 102 to DC power 104. Input rectifier 110 may be a simple diode bridge or other suitable active or passive rectification device capable of converting AC to DC. It is preferable to have rectifier 100 provide full wave rectification of the input AC power, however in certain embodiments the use of half wave rectification can be advantageous. The rectified DC power 104 created by the input rectifier 110 is further conditioned by a boost regulator 120. The boost regulator 120, also known as a step-up regulator, is a type of DC-DC power converter with a DC output voltage 106 greater than its DC input voltage 104. The boost regulator 120 is typically a switched mode converter, which includes various switching devices and control circuitry (not shown) to regulate the voltage of the rectified DC power 104 and produce a conditioned DC voltage 106. In some embodiments, such as the embodiment shown in FIG. 1, the converter 120 includes a power factor correction (PFC) component 125 to improve the power factor of the power source 100. PFC improves the overall efficiency of the supply 100 by compensating for harmonics and phase shifting caused by the input rectifier 110. The regulator 120 provides conditioned and voltage adjusted DC power 106 suitable for input to a resonant converter 130. Resonant converters 130 can achieve greater efficiency through the use of input power 106 that is substantially higher than the local mains voltage, often in the range of 450 volts. Boost regulator 120 is used to increase the mains voltage to the level needed by the resonant converter. In certain embodiments, the regulator 120 may be omitted, in which case the input rectifier 110, or other external DC source (not shown), will provide DC power 106 directly to the resonant converter 130.
A resonant converter is a type of switched mode DC-DC power converter that uses a resonant tank circuit comprising a combination of inductive and capacitive components, for energy storage, rather than a single inductor as is used in other switched mode supplies such as a Boost Converter. FIG. 2 shows a block diagram depicting a typical architecture for a resonant converter 200 as is known in the art. Resonant converter architecture can be divided into four main sections: a full or half wave bridge converter 202; a resonant tank 204; a rectifier 206; and an output filter 208. Starting from the input side, the full or half bridge converter 202 comprises a set of switches that chop the input DC voltage (VDC) to produce a square wave. A full-bridge converter 202 uses four switches to produces an AC square wave 210 with an amplitude of twice the input voltage VDC, while a half-bridge converter 202 uses only two switches to produce a square wave 210 with an amplitude of VDC and a DC offset of VDC/2. The switches in a bridge converter 202 are operated in complementary mode with a fixed duty cycle and some dead time. In a basic switched-mode power converter, such as the boost regulator 120 in FIG. 1, the output is typically controlled by adjusting the duty cycle of the bridge converter 202. Controlling the output by adjusting the duty cycle is known as Pulse Width Modulation control. However, in the case of resonant converters, control is achieved by adjusting the frequency of the bridge converter 202. Changing the frequency of the bridge converter changes the impedance of the resonant tank 204, thereby allowing control of power flowing to the output. The resonant tank 204 is made up of reactive components—capacitors and inductors—and can be arranged in several different configurations. A series LC resonant tank uses a resonant tank comprising an inductor connected in series with a capacitor, and has the resonant tank connected in series with the load. A parallel LC resonant tank also uses a resonant tank comprising an inductor in series with a capacitor. However the load is connected in parallel with the resonant capacitor. Another common configuration is the series-parallel LLC resonant tank, which has three energy storage components, a capacitor and two inductors, all connected in series (the ‘series’ portion of the ‘series-parallel’ designation), and the load is coupled in parallel with the second inductor (the ‘parallel’ portion). An LLC resonant tank circuit typically operates at high frequencies and can be highly efficient but has some difficulties when operating under no-load conditions. Various solutions to the no-load problems have been purposed, such as re-cycling the converter, but these solutions are difficult to control and reliability is a concern. The output 220 of the resonant tank 204 will have either a sinusoidal current or a sinusoidal voltage depending on the configuration of the resonant tank 204. A resonant inverter is created by combining a bridge converter 202 and a resonant tank 204 to convert a DC input voltage (VDC) to a generally sinusoidal alternating current (AC) output voltage 220. To complete the DC-DC resonant converter 200, a rectifier 206 and an output filter 208 are added to the resonant inverter to rectify and smooth the AC voltage output 220 created by the resonant inverter, yielding a DC output voltage Vout.
Resonant DC-DC converters 200, of the type described above, are used to provide DC power to various types of electroluminescent devices, battery chargers, or other devices requiring low level DC power, and because of their high efficiency they are widely used in drivers for LED arrays. These devices often operate in offices and homes, as well as other locations where safety is a concern, so they typically need to be approved by rating institutions such as Underwriters Laboratories (UL). Low level DC power supplies, such as the supplies used by LED arrays, are known as class 2 supplies and are defined by Underwriters laboratories as having transformer isolation and producing less than 60 volts DC. Among other requirements, UL approval requires electrical isolation between the input and output, as well as over-voltage protection (OVP) to prevent the output voltage form exceeding the specified maximum values. The OVP circuitry needs to constrain the output voltage during both normal operation and fault conditions. Over-voltage protection circuits are typically added to the final stages of the converter 170, for example the rectifier 176 or filter 178, by including additional components such as a crowbar circuit or a clipping circuit. Alternatively, OVP may be added by adding or enhancing a feedback controller. In any case, the addition of OVP comes with additional costs, including increased manufacturing costs, functional limitations, and/or reduced reliability.
Accordingly, it would be desirable to provide a resonant DC-DC converter that resolves at least some of the problems identified above.